(1) Field of the Invention
The present invention relates to a method for producing grain-oriented silicon steel sheets, and particularly relates to a method for producing grain-oriented silicon steel sheets having low iron loss without lowering their magnetic induction.
(2) Description of the Prior Art
Grain-oriented silicon steel sheets are required to have high magnetic induction and low iron loss. There have been proposed various methods for lowering the iron loss, for example, a method wherein a steel having a high Si content is used; a method wherein a product steel sheet having a small thickness is produced; a method wherein secondary recrystallization grains highly aligned to (110)[001] orientation, that is, to Goss orientation are developed; a method wherein secondary recrystallization grains having small size are developed; and the like. As the method for developing secondary recrystallization grains highly aligned to Goss orientation, there have been known, for example, a method disclosed in Japanese Patent Application Publication No. 15,644/65, wherein an Al-containing silicon steel sheet is cold rolled at a high final reduction rate; a method disclosed in Japanese Patent Laid-open Specification No. 12,614/77 and the like, wherein a silicon steel having a very small B content is used; a method disclosed in Japanese Patent Application Publication No. 13,469/76, wherein an Sb-containing silicon steel sheet is subjected to a secondary recrystallization annealing at a low temperature; a method disclosed in Japanese Patent Application Publication No. 38,652/81, wherein a cold rolled steel sheet having a final gauge is annealed at a temperature within the range of 600.degree.-650.degree. C. for 0.5-10 minutes before the steel sheet is subjected to a decarburization annealing; a method disclosed in Japanese Patent Laid-open Specification No. 151,423/83, wherein a cold rolled steel sheet is heated at a heating rate of 100.degree. C./min.-400.degree. C./min. within the temperature range of 600.degree.-700.degree. C. in the heating stage in the decarburization annealing to keep the rate of recrystallization of the steel sheet to about 50%; and the like.
According to these methods, the secondary recrystallized grains are surely and highly aligned to Goss orientation, and as a result a grain-oriented silicon steel sheet having high magnetic induction can be obtained but the secondary recrystallized grains have always coarse grain size, and the resulting grain-oriented silicon steel sheet still has not satisfactorily low iron loss.
While, when it is intended to develop secondary recrystallization grains having a small grain size, not only crystal grains aligned to Goss orientation, but also crystal grains deviated from the Goss orientation grow as secondary recrystallization grains. Therefore, the resulting grain-oriented silicon steel sheet has low magnetic induction and high iron loss.